This invention relates to pneumatic tires for vehicles and more particularly to belted radial ply tires of the type having a cap band overlying the belt thereof.
The expressions "radial tires" and "radial ply tires" as commonly used in the pneumatic tire art may be said to include various tire constructions having a carcass or body comprising one or more reinforcement plies of rubberized tire cords extending from bead to bead wherein the cords in each ply are substantially radial in orientation, i.e., the cords are oriented substantially normal to the beads and to the crown centerline of the tire. In a mono-ply radial tire the carcass or body cords normally have a 90.degree. bias angle, i.e., in the unshaped carcass they extend perpendicular to the planes of the beads. In a two-ply radial tire, the cords in each body ply are usually oriented at oppositely disposed small angles of up to 10.degree. with respect to the perpendicular to the bead planes, in which case the respective body plies are said to have oppositely disposed bias angles of 80.degree. or greater (but less than 90.degree.). In four-ply or heavier radial ply tire constructions similar opposed orientation of the cords in successive body plies is usually employed. All of these body or carcass constructions are contemplated within the scope and meaning of the expressions "radial" and "substantially radial" as used herein.
Radial tires are generally constructed with a breaker or belt interposed between the crown region of the carcass and the tread for reinforcing the latter, such breaker being comprised of one or more layers or plies of tire cords or cables which are generally inextensible, i.e., made of such materials as metallic wires, glass fiber, rayon, etc. In a mono-ply belt, the cords or cables have a relatively low bias angle of 0.degree., i.e., they are oriented substantially parallel to the planes of the beads and to the median equatorial plane, or crown center line, of the tire. If the belt is of a multi-ply construction, similar but opposed low bias orientations of the cords or cables generally at angles ranging up to about 25.degree.-30.degree. or so with respect to the median equatorial plane of the tire are employed in successive plies.
It is known that many types of tires, especially radial ply carcass tires in which, as above stated, the tread is reinforced by a belt or breaker composed of superposed, mutually crossed, rubberized plies of parallel, essentially inextensible cords or cables, often fail at high speeds because separations occur in the shoulder zones of the tires where the edges of the belt plies are located. Such ply separations are due to the cord ends at the edges of the belt plies becoming detached from the surrounding rubber under the combined effects of centrifugal force acting on the tire, flexing of the tire and heat build-up in the tire. This result is made even more likely by the fact that the cords or cables in the belt plies, being disposed obliquely to the median equatorial plane of the tire by virtue of said plies being cut obliquely with respect to the longitudinal direction of the cords or cables therein, have a natural tendency to spread apart at their cut ends. The edges of the belt thus constitute zones where the cut and free ends of the reinforcing elements, i.e., the cords or cables, by friction and by cutting, cause breaks both in the carcass plies and in the rubber of the tire.
Belted radial ply tires of the type presently known in the art are further subject to the disadvantage that they tend to snake or squirm in a direction orthogonal to the direction of motion of the vehicle on which they are mounted, which results in excessive tire wear, premature tire failure and excessive vibration. This snaking of tires cured in two-piece or non-segmental molds of the types known to the art, appears to be due, at least in part, to the fact that such tires must undergo their final expansion into the full mold contours after being inserted in the molds. During such expansion of a tire in the mold, the cords or cables of said belts pivot relative to each other, i.e. they pantograph to a lower bias angle than that at which the cords or cables are originally oriented in the belt plies during the building operation. Due to stresses within the tire and the uneven expansion of various portions of the tire, however, the cords or cables tend to pantograph non-uniformly, and it is the resultant non-uniform cord orientations which cause the aforementioned disadvantageous snaking.
It might be noted that belted radial ply tires having an additional band of textile cords overlying the belt are known in the art. For example, French Pat. No. 1,416,538 discloses the use of such a band having a single ply therein, the cords of which may be oriented at any desired angle from 0.degree. to 90.degree. to the median equatorial plane of the tire and which serves to shield the belt cords from moisture. This patent does not, however, disclose a structure which overcomes either the problem of tire distortion and ply separation at high speeds, or the problem of non-uniform pantographing of the cords or cables of the belt plies during tire manufacturing.
Another example of a belted radial ply tire utilizing an added band of textile cords over the belt is illustrated in U.S. Pat. No. 3,503,432. In that tire, the added band is made of heat-shrinkable nylon cords, and the shrinkage stresses developed in such nylon cords are utilized to condition the band to apply a compressive stress to the underlying belt plies and to absorb some of the stresses present in the tire during operation. The breaker and cap band portion of the tire must, however, be cured in a segmental mold and thus is already at its desired final diameter when being placed into the mold. Since there is no further in-mold expansion, therefore, the patent does not teach a solution to, nor even have to face the problem of, non-uniform pantographing of the belt cords during tire manufacture.